When projecting onto a flat screen, light that bounces off the screen propagates away from the screen towards the audience so the viewers observe the intended image. However, when projecting onto a concave screen, the image that is projected not only reflects back towards the audience, but also scatters towards other regions of the screen. Therefore, points on a concave screen can be illuminated not only by the intended direct light from the projector, but also light reflected from other regions of the screen not intended for that point. This other light ends up “polluting” the intended image and results in significant image degradation through loss of contrast and detail.
As used herein, the term “concave screen” might refer to any screen that is not essentially planar. A screen can be an object specifically designed to receive and reflect light from a projector or might be an object for another purpose that is used as a screen, such as a portion of a wall of a building. In any case, the screen is positioned to receive light from a projector (or the projector is positioned relative to the screen) and when light is projected onto the screen, the light (perhaps not 100% of it—some might be absorbed) is reflected and viewers in range of the reflected light see the image or images being projected by the projector. Concave screens include curved screens, piecewise planar, piecewise curved, and/or spherical, such that light projected into one point on the screen can reflect to another point on the screen.
The way light behaves is that it propagates in a straight line until encountering something nontransparent. For purposes of this description, atmospheric and refractive interference with light propagation can be set aside or ignored. When light encounters the screen, the reflected light need not reflect equally in all directions, but typically to provide a good experience to viewers independent of their location within a wide viewing angle, a screen is usually made such that light reflects over a wide solid angle from points on the screen. Because of this behavior, light reflected from an essentially planar screen will not intersect the screen, but with a concave screen light can intersect the screen at more than one bounce.
This second and subsequent reflection can be a problem. For example, to show an image of a dark, unlit passageway in a movie, the image projected from the projector might block all light where the passageway is to be displayed. Depending on the nature of how the image was captured, such as digitally or on film, that information is represented by the digital values for the blackest black or by film that totally blocks the projector lamp from shining on that portion of the screen. However, if the image is part of an outdoor daytime shot, the image might include a brightly lit wall near the passageway. Because of how the light behaves, the light from the projector for the brightly lit wall will be projected on that portion of the screen where the wall is to be shown, but then some of the light will reflect from there and intersect the portion of the scene where the dark, unlit passageway is to be shown. This leads to loss of contrast, i.e., the intended dark areas are not as dark as the artist would be if the screen were flat.
Some projection systems use numerical compensation, but this is often not a satisfactory solution. Numerical optimization is a computationally expensive, iterative process often requiring the reducing of the resolution of the image to obtain efficient solutions. See, for example, [Bimber-A] Bimber, O., Grundhofer, A., Zeidler, T., Danch, D., and Kapakos, P., “Compensating Indirect Scattering for Immersive and Semi-Immersive Projection Displays”, VR '06: Proceedings of the IEEE Conference on Virtual Reality, IEEE Computer Society, pp. 151-58 (Washington, D.C., USA, 2006) [cited as http://dx.doi.org/10.1109/VR.2006.34], [Mukaigawa] Mukaigawa, Y., Kakinuma, T., and Ohta, Y., “Analytical Compensation of Inter-Reflection for Pattern Projection”, VRST '06: Proceedings of the ACM Symposium on Virtual Reality Software and Technology, ACM, pp. 265-68 (Limassol, Cyprus, 2006) [cited as http://doi.acm.org/10.1145/1180495.1180549], and [Bimber-B] Bimber, O., “Projector-Based Augmentation”, Emerging Technologies of Augmented Reality Interfaces and Design”, ed. Haller, M. et al., pp. 64-89 (2006).
For arbitrary concave screen configurations, the compensation can be solved using a number of available numerical techniques shown by [Bimber-A], [Mukaigawa] and [Bimber-B]. Unfortunately, these techniques either require an intractable amount of memory storage for high-resolution projected imagery, or require iterative processing and resolution reduction of the projected images to obtain efficient solutions.
In view of the above, it would be desirable to improve contrast and a viewing experience for concave screens.